High temperature filtration of particulates has become an important component in many emerging technologies. Advanced coal conversion technologies, such as fluid bed gasification and combustion, benefit (if not depend) on removal of particulates at temperatures in the range of about 500.degree. to 1100.degree. C. Similarly, many of the new and diverse technologies known as cogeneration hold promise for the application of high temperature filtration. These applications range from gas cleaning for biomass gasification to power generation from the incineration of municipal solid wastes. The common technical thread among these applications is the need to remove particulates from gas streams at high temperature (about 300.degree. to 1100.degree. C.) so that process equipment (such as rotating machinery and heat exchange surfaces) will remain functional and efficient over a reasonable period of time.
A primary technology for high temperature filtration is the development of woven cloths composed of ceramic fibers. These materials can be fabricated into tube or bag filters that closely resemble conventional woven glass filter bags. Such ceramic woven cloth filters will give cleaning efficiencies that are adequate to satisfy both process equipment requirements and environmental constraints, and can be used at high temperatures and pressures.
A problem associated with the commercial viability of filtering technology using ceramic woven cloth filters is that of bag life or mean time to failure. This parameter has not been quantified to date in a statistically sound manner because of the difficulties associated with the conduct of testing at these hostile conditions. Much of the current concern with regard to bag life centers on the method of cleaning used to date. In high temperature, high pressure applications, the pulse jet method has been used exclusively for bag cleaning. This choice has been made because the method gives effective cleaning, involves no moving parts within the high temperature containment and is mechanically simple and inexpensive to implement. In this method, bags are pulled over a cylindrical support cage and this assembly is then supported by a tube sheet that separates the lower (dirty) side of the baghouse from the upper (clean) side. Dusty gas flows radially inward through the bag and exits the top of the bag into the clean side. Particulate is filtered on the outside surface of the bag resulting in a dust deposit or "cake" that grows in thickness with time. Eventually, the dust deposit causes a resistance to flow that is too high to be tolerated and the dust deposit must be removed. The pulse jet method requires that a pulse tube or orifice be located over the top of the bag and be connected to an external source of high pressure gas. Typically, a fast acting solenoid valve is used to administer a short, (25 to 100 ms) high pressure burst of gas down the inside of the bag. This pulse causes rather violent motion of the bag accelerating it radially outward to velocities that are typically 150 to 450 cm./sec. At a point when the bag is fully inflated and all of the fabric slack is taken up, the bag is rapidly decelerated as it vigorously snaps taut. It is this rapid deceleration that is thought to play the major role in separating the dust deposit from the surface of the filter media.
We have found this method to be very effective in regenerating clean surfaces on the bag which in turn, gives rise to very stable pressure drop performance of the bag system. The difficulty with this cleaning technique arises from the rather violent nature of the process and the high stresses that it imposes on the fabric of the filter media. In this process, considerable "hoop" stress is imposed on the fabric as it becomes taut. This creates fiber to fiber motion, rubbing and consequent abrasion as the fabric stretches at each pulse. This abrasion can lead to gradual loss of fabric strength and eventual failure. Subsequent to inflation, the bag is rather forcefully thrown back on the support cage surface as the flow through the bag resumes its normal direction from outside to in. During this process, there is potential for gradual fiber damage by rubbing against the support. Another phenomena occurs in pulse jet cleaning that can cause fabric damage. This is the eductor or venturi effect that occurs near the top of the bag. The high velocity of the pulse jet causes a low pressure at the bag top and actually results in inward suction during the pulse. At a point that is typically 15 to 30 centimeters down, there is a sharp transition from negative to positive pressure inside the bag with the bag surface being sucked in against the support above, and vigorously pushed out away from the cage below. This area is highly stressed, and subjected to rapid motion and sharp deformations all of which can compromise fabric life. This is especially true for high modulus, brittle fibers such as conventional glass and the ceramic fibers used in high temperature bags. A description of jet pulse cleaning and reverse flow cleaning of tubular filters is given in "How Hot Gas Cleaning Improves the Economics of Electricity-from-Coal", G. P. Reed, Filtration and Separation, March/April 1984.
It is an object of the present invention to provide a filter system wherein the cleaning of the filter bag or tube retains the mechanical simplicity and effectiveness of pulse jet cleaning while incorporating much gentler aspects of reverse flow bag cleaning.